A typical horizontal axis wind turbine is illustrated in FIG. 1. The wind turbine 1 comprises a tower 2, a nacelle 3 mounted on top of the tower 2 and a rotor 4 operatively coupled to a generator 5 within the nacelle 3. The wind turbine 1 converts kinetic energy of the wind into electrical energy. In addition to the generator 5, the nacelle 3 may house the various components required to convert the wind energy into electrical energy and also the various components required to operate and optimize the performance of the wind turbine 1. The tower 2 supports the load presented by the nacelle 3, the rotor 4 and other wind turbine components within the nacelle 3.
The rotor 4 includes a central hub 6 and three elongate rotor blades 7a, 7b, 7c of approximately planar configuration that extend radially outward from the central hub 6. In operation, the blades 7a, 7b, 7c are configured to interact with the passing air flow to produce lift that causes the central hub 6 to rotate about its longitudinal axis. Wind exceeding a minimum level will activate the rotor 4 and allow it to rotate within a plane substantially perpendicular to the direction of the wind. The rotation is converted to electric power by the generator 5 and is usually supplied to the utility grid.
The turbine blades have a root section at which it connects to the central hub. The root section is generally circular in cross section and for blades which are 80 m or more in length can be as much as 4 or 5 meters in diameter. At the opposite end of the blade to the root is the blade tip. The direction along the blade between the root and the blade tip is known as the span-wise direction. In the lateral direction, known as the chord-wise direction, the blade extends between a leading edge and a trailing edge.
FIG. 2 shows an example rotor blade construction, with the exploded perspective view in FIG. 2 showing the elements used in the construction of such a rotor blade. The rotor blade is formed from two half shells 202 and 206 which each comprise elongate reinforcing structures 204. The two reinforcing structures that extend substantially along the full length of the turbine blade from the root section to the blade tip are referred to as spar caps. The complete turbine blade is formed from the two half shells 202 and 206 and two shear web 205 placed in between. The shear webs 205 are used to couple together the spar caps in order to transfer shear forces.
Wind turbine blades, including the example shown in FIG. 2, are typically made out of fibre-reinforced plastics (FRP), and particularly glass-reinforced plastics (GRP), which may be formed as dry sheets and positioned into a mould to form the various layers of the blade shell. In order to reinforce the blade, a fibre with a higher strength factor or an increased number of layers of FRP may be used in particular locations that experience higher forces during operation. Using a higher strength fibre is often a more expensive option, and so it may be desirable to instead reinforce the blade structure with an increased thickness of FRP, particularly in the root section of the blade where the bending moment is at its maximum.
Increasing the number of layers of FRP increases the amount of time it takes to lay each of the individual FRP sheets into the mould. Each sheet of FRP must be carefully laid into the mould ensuring that there are no formation defects, such as bends, kinks or creases, in the sheets. Any bends, kinks or creases in the sheets of FRP will lead to a concentration of stress and will therefore reduce the strength of the fibre.
Using different material specifically for the blade root section also has its own problems because the sheets still need to be installed quickly and require the selection of the correct material along with accurate placement.
Existing methods of transporting sheet materials into position within a blade mould are not ideally suited for large scale manufacture. For example, the use of cranes to pick up and lift into place one or more sheets of material is not ideal because it is difficult to accurately position sheets with a crane. In addition, cranes can only be used to carry and position one sheet or stack of sheets at a time and are slow moving.
Thus, it is desirable to provide a method of manufacturing a wind turbine blade, using a mould, which reduces the amount of time required to install sheets of material, such as FRP, into the required position within the mould.